The addition of hydrogen halides to olefinically unsaturated organic compounds is well known to the art. It is also well known that these reactions are catalyzed by metals such as, for example, iron, cobalt, chromium, and nickel which are components of most pipes, processing equipment and storage tanks. Mixtures containing a hydrogen halide and one or more olefinically unsaturated organic compounds are commonly produced in large volume as by-products in a wide variety of industrial halogenation processes. Representative processes of this character are those involving halo-substitution of olefins or other unsaturated compounds with the concomitant formation of hydrogen halide, as well as those wherein the hydrogen halide is itself reacted with the unsaturated organic compound. These mixtures are seldom discarded since their components are valuable raw materials which are either recycled to the reactor or used in other chemical processes involving halogenated hydrocarbons.
A wide variety of compounds is known to the are to prevent hydrohalogenation reactions catalyzed by aluminum halides. These aluminum salts, due to their extreme reactivity, readily interact with and are destroyed by virtually any organic compound that contains a functional group. For example, U.S. Pat. No. 3,976,705 teaches the use of such diverse compounds as alcohols, aldehydes, organic acids, nitro-organic compounds, ketones, alkoxides, ammonia and hydrazine and additionally amines, amides, nitriles, thiols, sulfonic acids, phenols, esters, glycols and the like for preventing undesirable aluminum halide catalyzed hydrohalogenations. Aluminum halides also rapidly interact with a vast number of inorganic compounds. Since aluminum reacts, often violently, with and is severely corroded by hydrogen chloride and chlorinated hydrocarbons, even at low temperatures, it is not suitable as a metal for construction of processing equipment such as tanks, stills, heat exchangers, etc. which are exposed to these materials.
Process equipment designed for exposure to hydrogen chloride and/or chlorinated hydrocarbons is commonly fabricated from materials containing metals of the first transition series of the periodic chart, i.e., titanium, vanadium, chromium, manganese, iron, cobalt, nickel and copper. Numerous of methods are known to the art for reducing losses attributable to catalytically-induced interactions normally encountered when mixtures of hydrogen halide and olefinically unsaturated organic compounds are distilled, stored, transported, or otherwise maintained in the presence of metals of the first transition series.
In one such method the mixture is passed through water or an aqueous solution which selectively dissolves the hydrogen halide. While removal of the halide component of the mixture in this fashion is relatively simple, the step whereby anhydrous hydrogen halide is recovered from the aqueous wash solution is normally so expensive as to be economically unfeasible. In other separation methods the hydrogen halide is taken up by one or the other of a wide variety of chemicals in the form of a loose molecular adduct which is thereafter decomposed. This method, while expensive to operate due to high chemical and handling costs, is particularly effective when dealing with mixtures containing hydrogen fluoride, though it is not well adapted to the removal of the other hydrogen halides from such mixtures. The easiest method for separating the components of the mixture is by fractional distillation, the hydrogen halide usually coming off the top of the still and the unsaturated component being collected as bottoms. The drawback of this method is that whenever the separation is effected in metallic distillation columns, especially those made of a ferrous alloy, there ensues extensive hydrohalogenation of the unsaturated component of the mixture, it having been observed that this reaction is catalyzed by the salts which are formed in the column as the metal surfaces therein are attacked by acid. Even in the case of nickel-lined columns, considerable hydrohalogenation occurs once the liner has become so corroded as to give rise to the presence of appreciable amounts of nickel halide in the column.
In a somewhat different approach U.S. Pat. No. 2,615,791 discloses the use of nitriles and isonitriles to reduce the catalytically-induced reaction between hydrogen halides and olefinic compounds. While these nitriles and isonitriles are effective in controlling these reactions, they have several disadvantages in a practical application. They are highly toxic, making their handling both dangerous and expensive. They are also highly flammable and, when heated, emit toxic fumes. They can also react with steam to produce toxic vapors. A major process disadvantage of the nitriles and isonitriles is that they decompose under conditions of use to form solid hydrohalide salts. These solid salts can foul reboilers, heat exchangers, and other process equipment, resulting in expensive and time consuming plant shutdown.